Problem: 5 notebooks cost $9.50. Which equation would help determine the cost of 14 notebooks?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 14 notebooks. Since 14 notebooks cost $x$ , we have the following proportion: $\dfrac{14}{x}$ We can write the fact that 5 notebooks cost $9.50 as a proportion: $\dfrac{5}{\$9.50}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{14}{x} = \dfrac{5}{\$9.50}$